The Oldest Stars in the Galaxy – Searching for Metal-Poor Stars in the Galactic Bulge

Description

The nature of the first generations of stars that formed after
the Big Bang is one of the major topics in contemporary
astrophysics and cosmology. Such stars formed out of gas that was
relatively free of elements heavier than helium, during a time
before generations of supernovae had polluted the environment.
Previous hunts have found many of these ‘old’ stars in the
Galactic halo. However, not all parts of the Milky Way evolved at
the same rate. Cosmological...[Show more] models of galaxy formation have drawn
the conclusion that, due to the "inside-out" formation of
galaxies like the Milky Way, the oldest stars should today
preferentially reside close to the centre of the Galaxy – in or
near the Galactic bulge.
This thesis documents the initial results of the EMBLA Survey,
the first successful search for ancient and metal-poor stars in
the bulge of the Milky Way. By utilising the metallicity
indicator provided by SkyMapper photometry, we were able to
search through more than five million bulge stars, looking for
those rare objects with extremely low iron abundances. The
AAOmega spectrograph on the AAT gave us the opportunity to obtain
intermediate- spectroscopy of approximately 350 stars
simultaneously; during 24 nights of observing spread over three
years, we observed more than 14,000 candidate metal-poor stars to
spectroscopically confirm the photometric metallicity estimates.
We found that 49% of the stars observed with the AAT had
metallicities below [Fe/H]= 1.0, and around 1,000 stars with
[Fe/H]< 2.0. This is a remarkable achievement given that
previous searches had only found a total of 21 stars with
metallicities that low.
Thirty-seven of the most metal-poor stars were then observed with
high-resolution spectro- scopy using 8 m telescopes like Magellan
and the Very Large Telescope (VLT), to determine their detailed
chemical compositions. As part of the Gaia-ESO Survey, four stars
were observed in 2012 on the VLT, with metallicities of
2.72[Fe/H] 2.48. We then observed a further ten stars with
the MIKE spectrograph at Magellan in 2012, and in 2014 we
observed a final 23 targets.
We have found the first EMP stars in the bulge; nine of our stars
have [Fe/H]< 3.0, and one has [Fe/H]= 4.0. We compared the
abundances found in our sample with stars of the same
metallicities found in the Galactic halo, and found in general
similar trends. Unexpectedly, however, we only found one
carbon-enhanced metal-poor (CEMP) star (3%) while 20% of halo
stars with [Fe/H]< 2.0 are CEMP stars. In order to verify the
predicted old ages of our stars, we also investigated their
kinematics. We found that half the stars examined have tightly
bound orbits; remaining within the inner Galaxy rather than being
merely halo stars passing through the bulge region. This is
crucial, as the oldest stars are predicted to have the lowest
binding energies. The two most metal-poor stars in our sample
have binding energies low enough that there is a 50% chance they
formed at redshifts of z > 12, which would make them the oldest
known objects in the Universe.